Porous media are used in a wide range of industrial applications for filtering and dispersing gases and liquids. Typical examples of such uses include particle capture, flow restriction, sound attenuating, gas/liquid contacting, wicks, spargers, and atomizers. In the electronics industry, high efficiency filters are widely used to remove small particles from a variety of process streams. Sintered metal filters useful for this purpose and capable of removing more than 99.9999999% of particles having the most penetrating particle size, e.g., about 0.1 micrometer, from a process gas flow (i.e., having a log reduction value or "LRV" greater than 9) are illustrated in U.S. Pat. Nos. 5,114,447 and 5,487,771, and in co-pending applications Ser. Nos. 08/895,604 and 08/895,605, now U.S. Pat. Nos. 5,937,263 and 5,917,066, all of which are here incorporated by reference. A wide range of sintered metal media for use in the these various fields and applications, including high efficiently filters for the electronics industry, are available commercially from Mott Corporation, the assignee of this patent. Sintered porous ceramic media and polymeric membrane filters are also available commercially.
As described in the above referenced patents and applications, and as is well-known in the art, porous metal media are typically made by pressing or molding metal or metal-alloy powders of specified characteristics into a desired shape, e.g., a sheet, tube or cup. The shaped body is then sintered at high temperature to provide a porous element or media. Sintered porous ceramic media are made by similar known procedures.
The exact characteristics of a porous metal or other media are highly dependent on a number of factors, including the particular powder used, the green density, the sintering conditions employed, and the configuration of the media. Depending on the application, important physical characteristics of the media may include its resistance to corrosion (e.g., from reaction with a wide range of process gases and liquids), mechanical strength, and the ability to withstand high temperatures. A filter, for example, should provide a relatively high rate of fluid flow at minimum pressure drop, and must be capable of removing any particulate matter that could cause contamination in the downstream manufacturing process.
As is known in the art, sintered powder metal media are generally capable of providing the desired corrosion and high temperature resistance. However, and as also is well-known, the relatively high porosity needed to provide the desired flow at a low pressure drop often comes at the cost of low mechanical strength and a decrease in efficiency. The lack of strength of conventional porous media is most serious in the "green" form; poor handleability of "green" porous structures is a major concern in manufacturing procedures particularly for large dimension tubes and sheets.
Fine filamentary nickel powders, such as those produced by the carbonyl nickel process and sold by INCO, are being used commercially to create fine porous media. However, the "green" shapes made from these fine powders are relatively weak, and this is particularly so when the powders are more spherical than filamentary in shape. Moreover, fine powders have a high surface area and are highly active during the sintering process; thus, 10% to 15% shrinkage of the molded "green" shapes occurs when they are sintered at 1300-1700 degrees F., in either a protective atmosphere (such as hydrogen) or in vacuum, is common. Additionally, because these powders are typically sintered at low temperatures (e.g., 1300 deg. F.) and for relatively short times (e.g., 10 to 15 minutes) to maintain high porosity, even the final sintered structure is relatively weak since both the time and the temperature are less than that necessary to form strong sintered bonds. Thus, although such porous media are satisfactory for relatively small structures, severe limitations in the processing and mechanical properties of porous sintered metal structures have made it difficult to make larger structures, such as large sheets, continuous strips or tubes of substantial diameter or length. Porous ceramic media having pore sizes of the same order of magnitude as those of sintered metal powder filters have been made using fine oxide particles, e.g., of 1 to 5 micrometer size, but these ceramic elements tend to be considerable more dense, e.g., to have 55% to 75% of theoretical density.
There remains a need for a porous media having the highly desirable flow, heat and corrosion resistance characteristics of the best sintered powdered metal or ceramic media, but that is stronger, can be formed into larger structures, and that can provide greater overall flow at a low pressure drop. There is a particular need for structures which have a high "green strength", to facilitate handling and transfer during the manufacturing process.